My dream full-frame camera.

Again, and for the zillionth time, the camera I'd like will appear at some point.

For the squillionth time, it won't. It's a bonkers marketing proposition. It's dead easy to do technically. The same has been done for rangefinders, for instance the Epson RD1 (http://www.luminous-landscape.com/reviews/cameras/epson-rd1.shtml). The biggest problem is that all the available SLR bases are very low end (some of them Cosina made) or else the Canon 1V or Nikon F6 - which don't fit your spec. So, if you want a low end Cosina with a sensor in, no dice.

PS. Not quite, in fact. The Pentax K1000 is still made, so I suppose Pentax could make a full frame version of the K1000 some time. Low volume, it would have to cost much the same as any other FF camera, so $3k for a digital K1000. You'd like it, but they wouldn't sell many.